In barrel type engines, in which the cylinders are clustered around and parallel to a common axis, various mechanisms have been used to convert reciprocating piston motion to rotary output motion. Typical mechanisms include a nutating disc or swash plate, a universally pivoted rocking yoke, or other such member, coupled to a Z-shaped section in the output shaft. Wobble plate or yoke members have also been used with eccentric output elements coupled to straight shafts. In all these mechanisms the reciprocating to rotary drive structure is subjected to considerable loads and, in turn, often causes off-axis load to be applied to the pistons, resulting in uneven cylinder wear.
In structures in which the motion converting element is not mounted directly on the output shaft, the element is supported only by its connections between the pistons and the shaft. Without support the motion converting element can be moved off center by uneven loads, or by changes in load during acceleration and deceleration of the engine. This causes excessive wear on the various moving parts.